SUBPT

Subtract checked pointer

This instruction subtracts an optionally-shifted register value from a base address register value, and writes the result to the destination register. The optionally-shifted register value is treated as the offset.

If the operation would have generated a result where the most significant 8 bits of the result register differ from the most significant 8 bits of the base register, then the result is modified such that it is likely to be non-canonical when used as an address.

Encoding: Integer

Variants: FEAT_CPA (ARMv9.5)

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
1	1	0	1	1	0	1	0	0	0	0						0	0	1
sf	op	S									Rm								imm3			Rn					Rd

SUBPT <Xd|SP>, <Xn|SP>, <Xm>{, LSL #<amount>}

Decoding algorithm

if !IsFeatureImplemented(FEAT_CPA) then EndOfDecode(Decode_UNDEF);
constant integer d = UInt(Rd);
constant integer n = UInt(Rn);
constant integer m = UInt(Rm);
constant integer shift = UInt(imm3);

Operation

bits(64) result;
constant bits(64) base = if n == 31 then SP[64] else X[n, 64];
constant bits(64) offset = LSL(X[m, 64], shift);

result = base - offset;
result = PointerAddCheck(result, base);

if d == 31 then
    SP[64] = result;
else
    X[d, 64] = result;

Explanations

<Xd|SP>: Is the 64-bit name of the general-purpose destination register or stack pointer, encoded in the "Rd" field.
<Xn|SP>: Is the 64-bit name of the first general-purpose source register or stack pointer, encoded in the "Rn" field.
<Xm>: Is the 64-bit name of the second general-purpose source register, encoded in the "Rm" field.
<amount>: Is the left shift amount, in the range 0 to 7, defaulting to 0, encoded in the "imm3" field.